1. Field of the Invention
The present invention generally relates to communications filters. In particular although not exclusively the present invention relates to multiband cavity filters.
2. Discussion of the Background Art
Various forms of filters are employed in today's communications systems. Some of the more common types utilized are band pass, low pass, high pass and notch filters. A typical application of such filter types is within most household televisions and radios. Generally these devices employ band pass and low pass filters to select the desired station. Typically these tuning filters are constructed from conventional electronic components such as capacitors, inductors, resistors and operational amplifiers (in the case of active filtering).
While such filters are quite capable of handling transmissions in the AM, FM, VHF and selected UHF bands, they are not readily suitable for communications applications utilizing higher UHF frequency bands such as those used in microwave transmissions. At these higher frequency ranges some of the basic electrical characteristics of electronic components used in these filter constructions begin to degrade. This degradation alters the transfer characteristics of the filter causing distortion.
Accordingly, filtering in the higher UHF bands to EHF bands requires a different approach. One commonly used filter type for such higher bands, especially in high power communication systems is a cavity filter. Cavity filters are utilised in these high power systems due to their stability and their high Q factors.
One such use of a resonance cavity in a communication system is discussed in U.S. Pat. No. 2,337,184 entitled “Coupling Circuit”, which relates to a circuit for coupling a plurality of sources such as plurality of radio frequencies to a single load. A rectangular cavity resonator is coupled to a first transmitter, a second transmitter and a load, in this case an antenna. The cavity allows the two transmitters to utilize the antenna simultaneously without interference. The two transmitters excite two fundamental modes within the cavity,. the first mode being at the frequency of the first transmitter and the second being at the frequency of the second transmitter. The antenna is coupled to the resonator via a dipole p and is positioned in such a manner that it is excited equally by modes thereby allowing both modes to propagate through an antenna A.
U.S. Pat. No. 5,349,316 entitled “Dual Bandpass Microwave Filter” discloses a dual port bandpass filter. The filter consists of at least one resonance cavity having two independent modes of operation at displaced frequencies. This provides the filter with two independent passbands within the desired frequency band. In order to produce the two passbands the filter requires the incoming waveguide to be orientated at an angle to the filter such that both TE and TM modes are excited within the cavity, particularly the TE1,1,1 and TM0,1,0 modes.
Yet another form of dual mode cavity filter is discussed in U.S. Pat. No. 5,793,271. The filter in this instance is composed of one or more dual-mode resonant cavities. Each cavity produces two resonant modes at two different frequencies. The two modes have essentially the same field distribution but are orthogonal to each other. The cavity further includes a first set and a second set of tuning elements to tune the respective modes to the desired frequency.
One problem with the above discussed filter types is that they can be quite large and cumbersome. Furthermore the frequency tuning of such filters is relatively dependent upon the coupling tuning. This is the case with the filter of U.S. Pat. No. 5,349,316 which requires the signal coupling to be orientated at a certain angle in order to induce the required modes. This is not always possible and therefore the operation of the filter may be impaired.
Accordingly it would be advantageous to provide a multiband filter which is less obtrusive and provides for quasi-independent frequency and coupling tuning as well as providing an improved tuning arrangement.